[U-Boot] [PATCH 4/9] ARM: Implement non-cached memory support

[Stephen Warren]

On 08/18/2014 02:00 AM, Thierry Reding wrote:
> From: Thierry Reding <treding at nvidia.com>
>
> Implement an API that can be used by drivers to allocate memory from a
> poll that is mapped uncached. This is useful if drivers would otherwise

s/poll/pool/

> need to do extensive cache maintenance (or explicitly maintaining the
> cache isn't safe).
>
> The API is protected using the new CONFIG_SYS_NONCACHED_MEMORY setting.
> Boards can set this to the size to be used for the non-cached area. The
> area will typically be right below the malloc() area, but architectures
> should take care of aligning the beginning and end of the area to honor
> any mapping restrictions. Architectures must also ensure that mappings
> established for this area do not overlap with the malloc() area (which
> should remain cached for improved performance).
>
> While the API is currently only implemented for ARM v7, it should be
> generic enough to allow other architectures to implement it as well.

> diff --git a/README b/README

> +- CONFIG_SYS_NONCACHED_MEMORY:
> +		Size of non-cached memory area. This area of memory will be
> +		typically located right below the malloc() area and mapped
> +		uncached in the MMU. This is useful for drivers that would
> +		otherwise require a lot of explicit cache maintenance. For
> +		some drivers it's also impossible to properly maintain the
> +		cache. For example if the regions that need to be flushed
> +		are not a multiple of the cache-line size,

I would add:

*and* padding cannot be allocated between the regions to align them 
(i.e. if the HW requires a contiguous array of regions, and the size of 
each region is not cache-aligned),

 >                                                           then a flush of
> +		one region may result in overwriting data that hardware has
> +		written to another region in the same cache-line. This can
> +		happen for example in network drivers where descriptors for
> +		buffers are typically smaller than the CPU cache-line (e.g.
> +		16 bytes vs. 32 or 64 bytes).
> +
> +		Non-cached memory is only supported on 32-bit ARM at present.

> diff --git a/arch/arm/include/asm/system.h b/arch/arm/include/asm/system.h

> +#ifdef CONFIG_SYS_NONCACHED_MEMORY
> +void noncached_init(void);
> +unsigned long noncached_alloc(size_t size, size_t align);

s/size_t/unsigned long/ would match the arguments in the earlier patch 
to mmu_set_region_dcache_behaviour()'s prototype.

> diff --git a/arch/arm/lib/cache.c b/arch/arm/lib/cache.c

> +void noncached_init(void)
> +{
> +	unsigned long start, end, size;
> +
> +	end = ALIGN(mem_malloc_start, MMU_SECTION_SIZE) - MMU_SECTION_SIZE;

Not really "end" (which implies it's inside the region) but "first 
address outside the region". That said, I'm not sure how to express that 
succinctly, so perhaps "end" is fine!

> +unsigned long noncached_alloc(size_t size, size_t align)
> +{
> +	unsigned long next = ALIGN(noncached_next, align);
> +
> +	if (next >= noncached_end || (next + size) >= noncached_end)
> +		return 0;

If size is quite large, and next is near to U32_MAX, there's a chance of 
wrap-around there. Perhaps calculate the size left in the region 
(noncached_end - next), and validate that's >= size.